Generally, the invention relates to the field of computer memory backup devices which utilize magnetic tape media focusing upon techniques for sensing the position of such tape media. More specifically, the present invention focuses upon techniques for sensing the beginning-of-tape (bot) and end-of-tape (eot) positions in a perforated magnetic tape media.
Since the advent of programmable information processing systems, or computers, the need to store information has grown dramatically. This information storage is frequently accomplished through devices which interconnect with the computer and act relatively independently of it in response to signals received from the main data processing functions of the computer. These devices, known as peripheral devices, act to receive data from the main computer memory and then to store such data on a separate media within the peripheral device.
One of the aspects of typical memory devices and computer systems is their volatility--they can unintentionally loose their contents occasionally. To overcome this limitation, backup devices have evolved. Such devices serve the simple function of separately storing large amounts of data on relatively non-volatile media. In the field of these specialized devices, the use of magnetic tape media has greatly eclipsed all other types of uses. This use is so broad that magnetic tape cartridges which are specifically designed to be used with computer backup devices have been developed. Such cartridges specifically meet the needs of computer users in a standardized and optimum manner.
The present invention focuses upon standardized magnetic tape cartridge systems, such as are frequently used in computer tape backup systems. The American National Standards Institute defines "magnetic tape" as tape that "accepts and retains magnetic signals intended for input/output and storage purposes for information processing and associated systems." The "standardized" magnetic tape cartridge ensures that the use of the tape cartridge for computer memory backup will be optimal, as opposed to other types of media systems which might require accommodation of existing designs.
An aspect of utilizing magnetic tape media in cartridge form is that such media is designed as a continuous segment having a beginning and an end. Since magnetic tape is moved in front of the sensor at relatively rapid speeds (ranging in excess of 90 inches per second) the need for accurate sensing of the beginning and end of the tape is acute. Failure to sense either the beginning or end of the tape may result in damage to the tape media.
These circumstances have led to the development of several techniques for sensing the beginning and end of a magnetic tape. As one example, two distinct reflective markers are positioned at appropriate locations on the tape. A light source is then reflected off such markers and separately sensed to make the determination as to beginning or end of tape. In U.S. Pat. No. 4,410,148 to Dunlap, such a system is explained. In a second example, as shown in U.S. Pat. No. 4,763,217 to Oishi, transparent leaders are used. These transparent leaders allow the transmission of light beams through the beginning and ends of the tapes for appropriate sensing and determination. One of the more advanced ways of sensing tape position, and that involved in the present invention, is that of using aperture indications. Such aperture indications are actually small holes placed at specific points and in specific configurations at either extreme of the tape.
An example of the placement of such aperture indications is set forth in the American National Standard Institute (ANSI) specification applicable to computer tape cartridges. In this specification, standards which require these holes to have a diameter of 0.046 inches .+-.0.002 inches (1.17 mm.+-.0.05 mm) are set forth. The specification continues to add that these holes should be positioned in arrangements such that a light sensing means can be utilized.
In addition to setting forth such specifications, the ANSI standards advocate through example the use of a tungsten lamp or light emitting diode as a light source having a specific wave length emission. It is then suggested that this emission then be sensed through use of a silicon photo diode. This relatively simple method of sensing magnetic tape position has become firmly established for computer tape backup systems through the compliance with specifications and standards as they relate to the tape cartridges used in computer memory backup devices. In establishing standards, many of these specifications have actually acted to limit the techniques utilized by those skilled in the art of designing computer memory backup devices. Since the specifications set detailed standards which are widely accepted and adhered to, those skilled in the art have been led away from changes which might actually optimize many desirable aspects.
Among the aspects well known to be desirable are issues relating to maximizing sensitivity and minimizing energy consumption. As to the first, maximizing sensitivity, those skilled in the art of sensing tape position in magnetic tape backup devices have long felt the need to achieve better performance. The standards, however, have served to direct them away from the solutions discovered by the present invention even though the implementing arts have long been available. This was perhaps due to the fact that while those skilled in the art recognized the need for maximizing sensitivity, they failed to appreciate that one element of the problem lay in focusing upon the transmission of the light (or, more properly, electromagnetic radiation) through the aperture indications, not on the silicon photo diode or not at all. Instead, the specification implicitly, and other references explicitly have acted to teach away from the direction taken by the present invention. Those skilled in the art simply did not expect such advances to be possible without modification of the existing cartridges--and therefore the ANSI standard. In sharp contrast to the preconceptions developed and propounded, the present invention provides expanded capabilities with respect to accuracy, energy use, and efficiency in a system which adheres to the promoted standards.
In this regard, U.S. Pat. No. 4,848,698 to Newell, et al., acknowledges the use of ANSI standards as a source which dictates the positioning of the beginning-of-tape and end-of-tape indicia. Newell teaches the use of transparent leaders, but comments on the use of apertures (holes) in magnetic tape as they relate to the sensing of the tape beginning and end. Newell rejects this method as unacceptable because, he says, "holes must be made small or they weaken tape," but "holes must be large enough to distinguish from `pinholes`." The present invention overcomes these problems by accommodating existing "undesirable" aperture size, and by using a filtering method to distinguish "pinholes."